In U.S. Pat. application No. 515,120, filed Oct. 15, 1974, there is disclosed an electrically pumped, distributed feedback heterojunction diode laser which produces a highly collimated, polarized output light beam which is emitted perpendicular to the plane of the PN junction of the laser rather than through cleaved end faces in the plane of the PN junction. The diode laser includes a periodic structure buried at a heterojunction interface with the periodic structure producing the feedback necessary for lasing and, when the spacing of the teeth of the periodic structure is equal to an integer number of wavelengths of the light photons in the laser medium, the periodic structure provides an output beam perpendicular to the plane of the PN junction. In order to increase the intensity of this output beam, by limiting the light escaping from the sides of the laser, the external surfaces of the laser are cleaved and coated with highly light reflective films. Other laser devices, such as the optically pumped lasers disclosed in U.S. Pat. Nos 3,308,395 and 3,248,671, limit the escape of laser light by regulating the geometrical configuration of the active laser medium such that the light photons produced by the laser are internally reflected by striking successive faces of the laser medium at angles greater than the critical angle of the material of the laser medium until they arrive at an output area at a predetermined location on the surface of an edge of the laser medium.
In conventional solid state diode lasers, such as the laser of U.S. Pat. No. 3,308,395, the divergence of the output beam is large, a typical beam divergence in one direction being 10.degree. and in the other direction being 30.degree.-40.degree.. The electrically pumped solid state, distributed feedback laser of the aforementioned copending U.S. patent application produces an output beam having an angular divergence of only approximately 0.35.degree. in a direction along the length of the teeth of the periodic structure and of approximately 10.degree. in the direction across the teeth of the periodic structure. If the latter divergence could be reduced, the electrically pumped, solid state distributed feedback laser output beam would approach the very small angular divergence of a gas laser output beam and would be useful in many applications where gas lasers are now used.